If I'm not mistaken, vsftpd supports user-specific configuration options (ie configuration files for each user).
on my FC4 box shows the option:
user_config_dir
Enable that option, then create a config file with the same name as your admin user in /etc/vsftpd_user_conf/.
In the config file for your admin user, put a new set of allowed commands, this time including the "dele".
Another option would be to allow all user the "dele" command, but to limit their freedom by:
-chroot'ing them (and possibly not giving them the "rm" command)
-putting them in directories with sticky bit (makes deletion impossible for all users except owner = your admin user).